At present, with the increase of the speed and capacity of an optical transmission system, the optical phase modulation method, with the Differential Quadrature Phase Keying (DQPSK) as a representative, is training more attention in the industry. In the DQPSK modulation method, four different phases of optical wave represent different data signals, and its code element speed is only half that of the traditional optical amplitude modulation method, therefore, such modulation method has a low requirement on an optical device. Furthermore, the DQPSK modulation has better dispersion and polarisation mode dispersion performance than the amplitude modulation, so that it can be adapted to the lame-capacity and long-distance optical transmission system better.
The principle of the DQPSK modulation will be described below briefly. It is assumed that the optical carrier in the optical fibre communication adopting the DQPSK modulation way is represented as follows: Ei=Eexpj[ω0t+φ(t)], where E is field intensity, ω0 is the angular frequency of the optical carrier, and ω(t) is a modulation phase. The principle of the DQPSK modulation is: the information to be transmitted is encoded in a differential phase of a continuous optical bit and represented by Δφ, where Δφ is a value from [0, π/2, π, 3π/2]. It is assumed that the phase of the pulse of the k−1th optical bit is θ(k−1). If the subsequent bit is 0, 0, θ(k)=θ(k−1)+π, if it is 0, 1, θ(k)−θ(k−1)+π/2; if it is 1, 1, θ(k)=θ(k−1)+π; and if it is 1, 0, θ(k)=θ(k−1)+3π/2.
In a DQPSK modulation system, generally, a LiNbO3 modulator is required, however, the LiNbO3 modulator is very sensitive to the temperature and pressure due to its own material characteristics, therefore, a periphery control circuit is required to ensure the characteristics of the LiNbO3 modulator out of the influence of external factors and control it in order to implement a precise phase control in an actual system.
To ensure the accuracy of the modulation signal of the LiNbO3 modulator, now, the common control method mainly includes: (1) pilot signals of different frequencies are added to two arms of the DQPSK LiNbO3 modulator, and then, a backlight detection signal is collected to filter a difference frequency signal therein; when the difference frequency signal disappears, it may be thought that the LiNbO3 modulator is locked to a normal bias point. However, such method needs to introduce multiple pilot signals, besides, the detection result is the difference frequency signal, and the control loop is of high complexity, so that it is very difficult to implement the method. (2) A backlight detection signal is directly sampled to judge whether it contains a Radio Frequency (RF) harmonic signal at the same rate with the data bit stream; during the adjustment of the bias point of the LiNbO3 modulator, if the backlight detection signal contains no RF harmonic signal, it is thought that the bias point meets the requirement at the moment, and then the LiNbO3 modulator is locked normally. No pilot signal is required to implement such method, whereas, the data signal for the modulation is not the ideal digital signal with a sharp rising/falling edge, and harmonic signal noise may be introduced to influence the control, accordingly, the control accuracy is diminished.
Besides the LiNbO3 modulator, other modulators, which have multiple bias voltages and regulate the bias voltages for the Phase modulation of a signal, also have such problems as high complexity and poor accuracy, and now, there is still no effective solution for this.